1. Field of the Invention
The present invention relates to an optical displacement measurement device, and in particular to an optical displacement measurement device for directing light from a light source toward an object through an optical system which includes an objective lens, and detecting light reflected from the object using a detection section to thereby measure a displacement of the object relative to the objective lens.
2. Description of the Related Art
For non-contact measurement of an object displacement, optical probe methods, such as those described by Toyohiko Yatagai in “Applied Optics, Introduction to Optical Measurement” (4th version, Maruzen Co., Ltd., Mar. 15, 1992, p 120-124), are known. The optical probe methods described therein include a critical angle method, an astigmatic method, a knife edge method, a heterodyne method, and others. In any of these methods, a minute spot is caused to be formed on the surface of an object and light reflected from the object is collected and detected as for its condition. These methods can achieve highly sensitive detection, and devices which employ these methods can be made small and light-weight, such that they are suitable for use as, for example, an optical probe for optical measurement of surface roughness.
Further, Japanese Patent Laid-open Publication No. Hei 7-43148 discloses one automatic focusing technique referred to as a pinhole method. In a pinhole method, light reflected from an object and passing through an objective lens for light convergence is split into two light portions, and, while using pinhole plates which are respectively arranged ahead of the image point of one split light portion and behind the image point of another split light portion and optical detection devices provided immediately behind the respective pin hole plates, positional displacement between the object and the image point of the objective lens is detected based on outputs from the respective optical detection devices.
According to this automatic focusing technique, the object lens is controlled to move such that the light always focuses on an object. Thus, a displacement of the object can be measured based on the amount of movement of the lens.
Among these conventional non-contact methods for measuring an object displacement, an optical probe method, in particular, can attain a significantly high resolution. This method, however, suffers from a very narrow measurable range as a highly accurate detection range is limited to an area near the focal point. Specifically, accurate measurement is generally possible only within a range of a few μm, for example.
According to the automatic focusing technique and relevant improved technique disclosed in Japanese Patent Laid-Open Publication No. Hei 7-43148, control is made such that light always focuses on an object so that a displacement of the object can be measured based on the amount of movement of the lens. This method can realize a wider measurement range.
In an auto-focus operation, however, high speed measurement is not readily achievable should the surface of an object have discontinuous displacement due to a step, for example, as, in such a case, the lens would lose its direction to follow and another search operation may become necessary.
As described above, although a variety of non-contact displacement measurement methods have been proposed, these methods in conjunction with the advantages they provide, have remaining problems in view of their ability to obtain high precision, high speed, price reduction, usability, increased reliability, and so forth.
The present invention advantageously provides a readily operable optical displacement measurement device which employs a new method.